Welcome to brain stuff from How Stuff Works. Hey, I'm Christian Sager, and welcome to brain Stuff. If you have a car, then chances are you also have a rear view mirror. So imagine that one night you're driving home when some joker rolls up behind you with his high beams on, blinding you. Luckily, you're able to flip the tab on your rear view and make the mirror glare resistant, eliminating most of the light. But what's going on there? How can the same piece of glass have two different
amounts of reflection? This mirror's looks can be deceiving. See, your rear view isn't just one mirror, and it isn't flat. Instead, it's built in what's called a prismatic wedge shape, and it contains two reflective surfaces. There's a regular shiny mirror in the back, and there's a thin glass wedge right in front of it, closer to the driver. The front glass surface is at an angle to the back surface.
If you were to look at this mirror outside of its casing, it would be wedge shaped, with the thicker edge up at the top. During a daytime drive, the angled glass surface is pointing down and you're observing the reflection of the main mirror. But when you flip that little tab, the back mirrored surface usually points towards the dark ceiling. You don't see that image. What you see instead is the image reflecting off the front of the glass. The glass only reflects about four percent of the light
hitting the mirror, so it doesn't hurt your eyes. To prove that this is what's happening, take a flashlight with you one night and play with your mirror. Now not while you're driving, but you know, while you stopped. Flip the mirror into glare resistant mode and shine the light at the ceiling or sometimes try the floor. The fully reflected image will overwhelm the front surface reflections so you can see the ceiling. This is just the mechanism behind
the basic rear view mirror. More high end mirrors may have auto dimming electrochromic technology which uses a low voltage power supply to tint the glass. Or you might even drive a car with a rear view camera system. Check out the brainstuff channel on YouTube, and for more on this and thousands of other topics, visit how stuff works dot com.